The present invention is broadly related to a video camera and, in particular, a method and apparatus for compressing a level, and converting gradations, of a color video signal representing a color video picture of an object in high luminance areas without causing any change in hue of the picture.
FIG. 37A shows an ideal television system 300A consisting of a camera system, a recording system, a transmitting system and a receiving system. A picture is obtained by the camera system and sent via the recording system and transmitting system to the receiving system including a monitor for viewing, for example.
In the television system 300A, incident light of an object, such as a flower for example, passes through an objective lens 301, and is divided into red, green and blue components by a color separation prism 302. Each individual color component is then supplied to CCD solid-state image sensors 303R, 303G, 303B, so that a red image, a green image and a blue image of the object are obtained. The red, green and blue image signals are further supplied to a correlated double sampling (CDS) circuit 304 for appropriate processing, such as noise removal, resulting in red, green and blue color signals R, G, B.
Next, the color signals R, G, B outputted from the CDS circuit 304 are amplified in an amplifier 305 and are processed by a gamma correction circuit 306 and a signal processing circuit 307. The signal processing circuit 307 performs a well-known matrixing operation on the color signals R, G, B to obtain a luminance signal Y, a red color difference signal CR and a blue color difference signal CB. A sync signal is then added to the luminance signal Y, while the color difference signals CR and CB are modulated and combined to form a carrier color signal C. The luminance signal Y and the carrier color signal C are now ready to be recorded, for example, on a Video Tape Recorder (VTR) 308 of the recording system.
For distribution, for example, to a viewing audience via the transmitting system, the luminance signal Y and the carrier color signal C are reproduced by the VTR 308 for input to an encoder 309 which forms a video signal SV. The video signal SV is modulated in a modulator 310 to result in an RF signal, which is then transmitted from a transmitting antenna 311. The RF signal received by a receiving antenna 312 of the viewing audience is demodulated in a demodulator 313 such that the video signal SV is recovered.
The receiving system performs virtually inverse operations with respect to the corresponding operations of the transmitting system. Namely, the luminance signal Y and the carrier color signal C are recovered from the video signal SV by a decoder 314. Then, the luminance signal Y and the carrier color signal C are supplied to a signal processing circuit 315 where the carrier color signal C is demodulated to obtain the color difference signals CR and CB. The luminance signal Y and the color difference signals CR, CB are processed to form color signals R, G, B. Thereafter, the color signals R, G, B outputted from the signal processing circuit 315 are supplied to a cathode-ray tube (CRT) 316, and the picture of the object-in-interest (the flower) is displayed on the CRT 316.
Although a nonlinear device exists in the signal line of this ideal television system 300A, namely the CRT as known in the art, the entire process starting from the object and ending with the picture display is linear as viewed by the audience. This is due to the presence of the gamma correction circuit to compensate for the CRT non-linear operation, whereby the picture of the object is accurately reproduced for viewing.
As stated above, the described television system is ideal, without any limitations or restrictions. In practice, however, the dynamic range of each image sensor 303R, 303G, 303B is limited. In addition, the recording and transmitting systems have operational restrictions on signal recording and transmission in order to conform to accepted standards. Hence, it is virtually impossible to achieve the configuration of FIG. 37A. The standards imposed on the signal recording and transmission are determined to be very restrictive, and therefore adequate measures are needed to include a wide dynamic range of the incident natural light within the prescribed range as allowed by the standards.
For this reason, in a practical television system 300B, a pre-knee circuit 321 is inserted between an amplifier 305 and a gamma correcting circuit 306 as shown in FIG. 37B. Further, a knee circuit 322 is inserted between the gamma correcting circuit 306 and a signal processing circuit 307. This is done to fit the levels of the color signals R, G, B into the prescribed range of the standard by providing nonlinear input-output characteristics of the knee circuit. Since the signal levels according to the broadcasting standards refer to the color signals R, G, B, it is possible to conform with those standards by processing the color signals directly. In FIG. 37B, any components corresponding to FIG. 37A are designated by like reference numerals.
According to the system of FIG. 37B, the color signals R, G, B are non-linearly processed--each signal being processed independently from other signals--without corresponding inverse operations to compensate for this non-linear processing. The complementary operations are disrupted between the gamma correcting circuit 306 and the gamma characteristics of the CRT 316. As a result, luminance and hue of the picture of the object displayed on the CRT are different from the actual luminance and hue of the image of that object as perceived by the human eye.
Although the knee compression is determined to be the best operation for compressing the dynamic range of the incident light such that harmful effects are minimized on the object reproduction, a defect due to the knee compression occurs making an undesirable change in hue visually perceptible and unpleasant. For example, in a person's portrait shot in a slightly bright area, the problem occurs making that person look unhealthy with the skin color appearing more yellow than should be.
A need therefore exists for a method and device that overcome the above disadvantages.